Re: [PATCH for-7.1] hw/mips/malta: turn off x86 specific features of PIIX4_PM

["Philippe Mathieu-Daudé via" <qemu-devel@nongnu.org>]

On 28/7/22 15:16, Igor Mammedov wrote:
> On Thu, 28 Jul 2022 13:29:07 +0100
> Peter Maydell <peter.maydell@linaro.org> wrote:
> 
>> On Thu, 28 Jul 2022 at 12:50, Igor Mammedov <imammedo@redhat.com> wrote:
>>>
>>> QEMU crashes trying to save VMSTATE when only MIPS target are compiled in
>>>    $ qemu-system-mips -monitor stdio
>>>    (qemu) migrate "exec:gzip -c > STATEFILE.gz"
>>>    Segmentation fault (core dumped)
>>>
>>> It happens due to PIIX4_PM trying to parse hotplug vmstate structures
>>> which are valid only for x86 and not for MIPS (as it requires ACPI
>>> tables support which is not existent for ithe later)

We already discussed this Frankenstein PIIX4 problem 2 and 4 years ago:
https://lore.kernel.org/qemu-devel/4d42697e-ba84-e5af-3a17-a2cc52cf0dbc@redhat.com/
https://lore.kernel.org/qemu-devel/20190304210359-mutt-send-email-mst@kernel.org/

>>> Issue was probably exposed by trying to cleanup/compile out unused
>>> ACPI bits from MIPS target (but forgetting about migration bits).
>>>
>>> Disable compiled out features using compat properties as the least
>>> risky way to deal with issue.

So now MIPS is forced to use meaningless compat[] to satisfy X86.

Am I wrong seeing this as a dirty hack creeping in, yet another
technical debt that will hit (me...) back in a close future?

Are we sure there are no better solution (probably more time consuming
and involving refactors) we could do instead?

Thanks,

Phil.

>>> Signed-off-by: Igor Mammedov <imammedo@redhat.com>
>>
>> Resolves: https://gitlab.com/qemu-project/qemu/-/issues/995
>>
>>> ---
>>> PS:
>>> another approach could be setting defaults to disabled state and
>>> enabling them using compat props on PC machines (which is more
>>> code to deal with => more risky) or continue with PIIX4_PM
>>> refactoring to split x86-shism out (which I'm not really
>>> interested in due to risk of regressions for not much of
>>> benefit)
>>> ---
>>>   hw/mips/malta.c | 9 +++++++++
>>>   1 file changed, 9 insertions(+)
>>>
>>> diff --git a/hw/mips/malta.c b/hw/mips/malta.c
>>> index 7a0ec513b0..0e932988e0 100644
>>> --- a/hw/mips/malta.c
>>> +++ b/hw/mips/malta.c
>>> @@ -1442,6 +1442,14 @@ static const TypeInfo mips_malta_device = {
>>>       .instance_init = mips_malta_instance_init,
>>>   };
>>>
>>> +GlobalProperty malta_compat[] = {
>>> +    { "PIIX4_PM", "memory-hotplug-support", "off" },
>>> +    { "PIIX4_PM", "acpi-pci-hotplug-with-bridge-support", "off" },
>>> +    { "PIIX4_PM", "acpi-root-pci-hotplug", "off" },
>>> +    { "PIIX4_PM", "x-not-migrate-acpi-index", "true" },
>>> +};
>>
>> Is there an easy way to assert in hw/acpi/piix4.c that if
>> CONFIG_ACPI_PCIHP was not set then the board has initialized
>> all these properties to the don't-use-hotplug state ?
>> That would be a guard against similar bugs (though I suppose
>> we probably aren't likely to add new piix4 boards...)
> 
> unfortunately new features still creep in 'pc' machine
> ex: "acpi-root-pci-hotplug"), and I don't see an easy
> way to compile that nor enforce that in the future.
> 
> Far from easy would be split piix4_pm on base/enhanced
> classes so we wouldn't need x86 specific hacks in 'base'
> variant (assuming 'enhanced' could maintain the current
> VMSTATE to keep cross-version migration working).
> 
>>> +const size_t malta_compat_len = G_N_ELEMENTS(malta_compat);
>>> +
>>>   static void mips_malta_machine_init(MachineClass *mc)
>>>   {
>>>       mc->desc = "MIPS Malta Core LV";
>>> @@ -1455,6 +1463,7 @@ static void mips_malta_machine_init(MachineClass *mc)
>>>       mc->default_cpu_type = MIPS_CPU_TYPE_NAME("24Kf");
>>>   #endif
>>>       mc->default_ram_id = "mips_malta.ram";
>>> +    compat_props_add(mc->compat_props, malta_compat, malta_compat_len);
>>>   }
>>>
>>>   DEFINE_MACHINE("malta", mips_malta_machine_init)
>>> --
>>> 2.31.1
>>
>> thanks
>> -- PMM
>>
>